A number of diphenylhydantoin (DPH) derivatives were synthesized and their anticonvulsant properties were evaluated by the maximal Electroshock Seizure test described by Swineyeard et al., in 1952. Substitution of one of the phenyl rings of DPH with halogen (4-CL or F) or substitution of all the halogens in one or both of the phenyl rings (d5 or d10) did not markedly alter the ED50 values. However, substitution of both the phenyl rings with halogen (4,4'-C1,C1 of F,F) markedly altered the ED50 values. Results obtained in this study indicate that substitution of the phenyl rings of DPH prolongs therapeutic efficacy and decreases acute neurological toxic effects.